Just remove the rubber eyecup. It'll allow your eye to get closer. However, it is worth checking to see if the plastic around the eyepiece is abrasive to your glasses (particularly plastic lenses, or special coatings). Rub the exposed eyepiece frame briskly on a remote corner of your vision-correcting lens, or on an old pair of glasses.
The two virtual images are projected at different distances. Try borrowing a technique from microscopy. That is, look through the viewfinder with both eyes open! By doing so, you may be able to "fool" your viewfinder eye to focus on a farther point; one that may be closer to the distance of the images in the viewfinder.
It may also be worth trying out some diopter-adjustment lenses for the viewfinder. These are usually selected by trial-and-error, so you might have to search high and low for a Canon dealer with a good selection of them in stock.
It blocks stray light from entering viewfinder when your eye is not up to the eyepiece--like when shooting from a tripod. Since the exposure is determined from light bled from the viewfinder image, light entering the pentaprism could find its way to the exposure sensor. This could cause underexposure.
There is nothing terribly special about this piece. If you lose it, just remember to cover the eyepiece with your hand or something else opaque when the exposure reading is being taken. (Mike Coren points out that caps from Kodak film canisters can be substituted in a pinch!)
The dust is usually on the mirror or the focusing screen. If you view your camera with the lens off in the right kind of light, it should be visible to the naked eye.
Just take one of those blower bulbs and force some air over the mirror and screen (DON'T used compressed air NOR solvents!). If it's really persistent, you might try using a very fine brush, but I'd only do that if I were really annoyed, since the dust in the viewfinder doesn't affect the image on film (except in the RT), and you can damage the mirror and screen by touching them.
The screens are pretty easy to change. They each come with a little tool that is shaped like a screen on a handle. There are little "arms" which allow the screen to be gripped from the inside of the metal frame in which the focusing screen is mounted. On the handle, there is a small lever that releases the screen from the body when pressed.
So, the operation is to: 1) Insert tool into existing screen so that the "grabber arms" take hold. 2) Release the screen by pressing the lever. 3) Set the screen into the screen holding case. 4) Grab the new screen from the screen holding case. 5) Insert into body. (There should be a "click" when it is locked in.) 6) Remove tool.
It's much easier to understand from the instruction sheet (since it has diagrams). In any case, this method seems more foolproof than the tweezers method used on the EOS-1 and other bodies.
The only trouble with installing a screen (EOS or otherwise) is dust. If you get dust between the screen and the pentaprism, you have to remove the screen, etc. (Well, I guess there is also the worry of dropping the screen and destroying it, but the tool has been pretty reliable.)
As far as I can tell, every focusing screen comes with the same instruction sheet, and there are no warnings regarding exposure compensation. Canon say that no exposure compensation is required under normal conditions.
Subjectively, it seems that the lighter the focusing aid looks (like in the double crosshair), the darker the rest of the screen looks. So, Canon may be compensating by adjusting the darkness of the surrounding matte. (But this is all speculative.)
Note however that there may be exposure problems under unusual circumstances, such as when using the FD->EOS macro adapter. Metering considerations may require the use of only certain screens (see sections 9.18 and 4.3).
For general shooting, the "cross-split" (Type "L") seems most useful since it gives feedback as to how the focus needs to be adjusted. It is also more versatile than the "new split" (Type "B") in that it works the same both horizontally and vertically, and allows one to use detail in two directions instead of one.
For slow lenses, I do not recommend the "microprism" (Type "A"). It is hard to line it up in a way that avoids the little microprisms from going dark. It also gives no hints as to how the focus needs adjusting.
The Type "B" screen seems to be quite effective with lenses as slow as f/8-f/9. If you keep your eye in the center of the viewfinder the split part of the screen does not "black out". It may even be usable at f/11. Canon specifically state that this screen is usable with lenses slower than f/5.6. Canon also confirm that its performance in this regard is better than that of the Type "L" screen.